The present invention relates generally to the field of imaging devices, and more particularly to the improvement of noise immunity within the structures of CMOS image sensors.
Solid state image sensors, such as charged-coupled devices (CCD) and CMOS (complementary metal oxide silicon field effect transistor) image sensors (CIS) are commonly used as input devices for electronic video and still cameras, robotic/machine vision, and etc. These sensors are comprised of a light sensing element (photodiode) within individual pixels that are arranged into two-dimensional rows and columns as pixel arrays. The light data captured by the plurality of the light sensing pixels are together processed using associated logic and amplifying circuits to generate a whole optical image as sensed by the pixel array. Each pixel may include a color filter located over the light sensing element to selectively process image data by a specific color. Conventional solid state CIS devices are designed such that the incoming light beams are focused by a microlens through a color filter layer such that they converge along the focal axis of the microlens to strike the light sensing element (photodiode). The photoenergy from the light beams, upon striking the photodiode, frees electrons within the photodiode to be processed as the pixel's image data. However during actual operation of the image sensor, stray and undesired light beams not directly representative of the intended image capture, may strike the microlens at oblique incident angles. At certain oblique angles and light intensities, the stray and undesired light beams may manifest as electrical noise data generated by the photodiode to adversely affect the processed image. As result, the noise data degrades the accuracy and quality of the captured and processed image.
A major source of stray and undesired light beams occurs at the dimensional boundary edges of the active pixel array. At these boundary edges the incoming light beams may strike nearby non-light sensing peripheral structures and become diffracted back onto the pixels' microlens as stray and undesired light beams. These peripheral structures may include the support decoder, timer, amplifier circuits, data bus, and contact structures used for the image processing of the electrical data generated by the light sensitive pixel array. Conventional color CIS devices commonly use the application of semi-opaque light shielding layers to cover these peripheral structures located outside of the light sensitive pixel array. The light shield layers are very effective in reducing most of the light diffracted back onto the pixel array. However, the conventional light shield layers do not totally block the incoming light. As new device and process technologies advance, the light sensitive pixel arrays become geometrically smaller and denser. The smaller, denser pixel arrays are more susceptible to the noise issues and problems generated by stray and undesired light beams.
What is desired is an improved method for the enhancement of noise immunity within the image sensors. Such an improved method would further minimize or eliminate the stray and undesired light that could cause the noise generated within the pixel arrays.